The study has 2 arms:

Arm A (Cohort A) is a nonsurgical arm (N=16) that will serve as the primary study cohort and evaluated for the primary study endpoint. Subjects in this arm receive a single priming dose of both INCMGA00012 and INCAGN01876 prior to stereotactic radiosurgery (SRS), then undergo SRS (8 Gy x 3 fractions). Following SRS, INCMGA00012 (IV every 4 weeks) and INCAGN01876 (IV every 2 weeks) are resumed and continued until disease progression, unacceptable toxicity, or for 2 years, whichever occurs first.

Arm B (Cohort B) is a surgical arm (N=16) that will allow for evaluation of the effects on the tumor immune microenvironment of INCMGA00012, INCAGN01876, and SRS. In order to be enrolled on this arm, subjects must have a clinical indication for surgical resection of the recurrent GBM tumor. Prior to planned surgical resection, subjects receive neoadjuvant immunotherapy (one of two possible combinations, as outlined below). Subjects then undergo surgery. Postoperatively, the immunotherapy combination of INCMGA00012 (IV every 4 weeks) and INCAGN01876 (IV every 2 weeks) is resumed and continued until disease progression, unacceptable toxicity, or for 2 years, whichever occurs first. Subjects in the surgical arm with a focus of contrast-enhancing tumor that is amenable to SRS will be assigned to surgical sub-arm #1 of Cohort B (N=8). These subjects will receive neoadjuvant INCMGA00012 + INCAGN01876 + SRS. All other subjects enrolled on Cohort B (N=8) are treated with neoadjvuant INCMGA00012 + INCAGN01876 (without SRS).